Flocculant requirement

In most applications, the amount of polymer used is just sufficient to flocculate a sample of the feed. The amount necessary, as assessed in the laboratory, is generally the amount used in practice on the centrifuge, plus or minus a small fraction. However, recently there has been considerable development in decanters and their use in obtaining extra-dry cakes from compressible sludges, particularly effluents. In these instances, the consumption of polymer has increased considerably. 

The amount of flocculant needed increases as the extent of dryness required in the cake increases, and it increases exponentially. The amount of flocculant required also increases with the feed rate to the centrifuge [12]. In practice, on a dry solids application, the polymer used will be two to three times that which would be used on a standard application with the same process material. 

There has not been a theoretical formula proposed to quantify polymer demand. However, the available data suggest a format similar to equation (4.59)  

Practical data can be very erratic, as it is easy to overdose when striving for extra dryness. When assessing the minimum polymer requirement, it is necessary carefully to adjust all operating parameters, to ensure performance is at the limit, without contingency levels added. 

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Thus, the situation is currently clouded by an incomplete understanding of both the kinetic processes and the equilibrium behavior of polyelectrolytes. For specific systems, consistent correlations have been constructed, but a general understanding of the functioning of polymeric flocculants requires more extensive fundamental experiments and development of the equilibrium theory for polyelectrolytes. 

Coagulants may be either organic such as polyelectrolytes or inorganic such as alum. Coagulants can be used alone or in conjunction with flocculants to improve the performance of the flocculant or reduce the quantity oF the flocculant required. In some systems, where a flocculant has been used in an upstream process, a coagulant may be needed to allow additional flocculant to be effective. 

In the screen tests, each coagulant or flocculant is added to the beaker samples of representative slurry or hquor in a dropwise fashion, while the sample is mixed with a spatula, stirrer, or 3-6 jar stirrer mechanism. The amount of coagulant or flocculant required to initiate floe particle formation is noted along with relevant notes as to the size of the floe, capture of fines, resultant liquor clarity, and stabihty of the floe structure. The dosage is typically noted in g/t solids if the sample is primarily solids (thickener design), or in mg/L liquor if the sample is primarily for clarification and the solids concentration is low. 

The two terms agglomeration and flocculation require some clarification. Agglomeration is defined as a gathering of smaller particles into larger size units. 

Wktor Oecon of water should only be under taken by trained water purification per sonnel. a BoN smaN amounto 15 minutes, a Chlorinate using chlorination kit. a Add lodlna water purification tablets to small amounts of water. e Flocculation (requires special chamlcate Id ternou suspended matter in water). e ton exchange Removes radiotons from solution). 

PSD measurements were also used in the selection of design criteria for granular-media filters. One design problem evaluated was the duration and intensity of flocculation required prior to filtration. Results of... 

The mechanism of protein casse in wine is usually (Ribereau-Gayon et al., 1976) included in the general diagram of flocculation of a hydrophihc colloid (Section 9.3.2). Flocculation requires the disappearance of two stabilizing factors charge and hydration. 

THEORIES OF DEPLETION STABIUZATION AND FLOCCIILATION 395 minimum can achieve values sufficient in magnitude to impart stability or induce flocculation. As flocculation requires a well depth of only a few ksT whereas stability requires, say, 10 times this value, depletion flocculation will be induced at a lower concentration of polymer than is depletion stabilization. 

The number of antiflocculants suitable to stabilize emulsions is limited and the effect is less distinct compared with polymeric particles. Selection of an anti-flocculant requires consideration of net charge increa.se, effects of pH shift, and toxicological acceptance (38). 

If laboratory tests show that flocculation is difficult, impossible, or requires excessive quantities of flocculant, then a primary coagulant may be considered. This adds an extra dimension to the laboratory tests and will invariably require flocculant reassessment due to the resultant change of pH. Quite often, the ionic activity of the flocculant required would change, from cationic to anionic or vice versa. 

The measurement and evaluation of flocculation state is typically carried out over 10 min by means of programmed dosing, in which the flocculation value F is recorded during a stepwise increase in flocculant dosing. Figure 4 shows an example of a flocculation curve. The amount of flocculant required for optimal flocculation is that where F reaches an asymptotic limit (bridging mechanism) or a maximum (patch charge mechanism). 

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